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EXPLAINER: What is a storm surge and why is Denmark experiencing a 'once a century event'?

Michael Barrett
Michael Barrett - [email protected]
EXPLAINER: What is a storm surge and why is Denmark experiencing a 'once a century event'?
A footbridge submerged in water at Sandersvig in Denmark on Friday October 20th. Photo: Mads Claus Rasmussen/Ritzau Scanpix

Denmark is currently experiencing severe weather, the main feature of which is ‘storm surges’ which have been described as the worst the country has seen for 100 years. What are they and why are the current ones so bad?

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Strong easterly winds are causing high water levels and storm surges in Denmark, with the worst of the weather forecast today and tomorrow.

Some parts of the coast line could see waters reach their highest level for 100 years, senior consultant Poul Jensen of the Danish National Hazards Council told news wire Ritzau on Friday.

“The water level will actually be well over a 100-year-event in several places. We’ve not seen it yet, so it’s just alerts we’re talking about [so far],” Jensen said.

National Met office DMI has meanwhile issued a category 3 “very dangerous” alert for the affected areas, and local police have advised some residents to leave.

READ ALSO: How transport in Denmark is affected by storms and flooding this weekend

What is a storm surge?

The National Geographic describes a storm surge as “a rise in sea level that occurs during tropical cyclones, intense storms also known as typhoons or hurricanes. The storms produce strong winds that push the water into shore, which can lead to flooding.”

This makes storm surges “very dangerous for coastal regions”, it states.

The surge itself is caused by the relationship between winds and the surface of the sea. Water is pushed in the direction the wind is blowing, raising the sea level. The amount the water level rises by will increase as the strength of the wind increases.

Several other factors can affect the phenomenon, including atmospheric pressure and the tide. Additionally, water will more easily flood shallow coasts than deeper ones.

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The Danish Coastal Authority explains that high waters, which occur due to strong winds, become storm surges if they result in flooding.

“With strong onshore winds, the water level along a long, straight coast will rise in line with the strength and duration of the wind,” the Danish Meteorological Agency writes.

“In more enclosed sea areas, coasts that are not directly exposed to onshore winds can also be affected by storm surges. Initially, it is the coast that is affected, including harbours. But if the water level gets high enough, seawater can penetrate far inland,” it says.

The situation can be particularly dangerous for areas which lie below normal sea level and are protected by coastal defences. If the water level overflows or breaks through the coastal protection, the land behind will be flooded very quickly.

In such situations, it may be necessary to evacuate the entire flooded area to prevent a life-threatening situation.

READ ALSO: What can homeowners in Denmark do to protect houses from high water damage?

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100-year event

A 100-year-event as referred to by Jensen means something that occurs so rarely you’d only expect to see it once a century.

Statistics showing the sea level over time are kept by the Danish Coastal Authority.

The southern part of the Little Belt strait is likely to be the worst-hit by the current storm surges, with sea levels between 1.9 metres and 2.4 metres over normal.

South Jutland town Haderslev, for example, could see water levels top out at 2.14 metres over normal at 1:30am on Saturday

If the forecasts are borne out on it would represent a 100-year-event, according to Danish Coastal Authority records. That is because a sea level rise of 1.75 metres is considered to be a once-in-a-century event.

One reason for this is the relatively rare nature of a strong easterly onshore wind, Jensen said.

DMI bases its forecasts on a model which calculates the likely sea level based on wind strength and direction.

The current conditions mean the model “is accurate almost to the centimetre”, Jensen said.

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